Solid State Ionics 134 (2000) 119–125www.elsevier.com/ locate / ssi

The effect of pulse duration and oxygen partial pressure onLa Sr CoO and La Sr Co Fe O films prepared by0.7 0.3 32d 0.7 0.3 0.2 0.8 32d

laser ablation

a , b a b*David Waller , Luigi G. Coccia , John A. Kilner , Ian W. BoydaDepartment of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BP, UK

bDepartment of Electrical and Electronic Engineering, University College, Torrington Place, London WC1E 7JE, UK

Abstract

The effect of oxygen pressure and laser pulse duration on the production of thin films of La Sr CoO (LSC) and0.7 0.3 32d

La Sr Co Fe O (LSCF) by laser ablation has been investigated. Evidence of reduction and phase separation of the0.7 0.3 0.2 0.8 32d

target surfaces was found by X-ray diffraction. Ablation of LSC, at low oxygen partial pressures and with an excimer laser,led to the production of poor quality films, compared with the LSCF. This is attributed to the greater stability of LSCF interms of low oxygen pressures. 2000 Published by Elsevier Science B.V.

Keywords: Laser ablation; Oxygen pressure; Laser pulse duration; LSC; LSCF

1. Introduction technique for the production of complex oxides.These materials are readily vaporised using UV

Mixed-conducting oxides such as lasers, and the cation stoichiometry is maintainedLa Sr CoO and La Sr Co Fe O , both provided the pulse duration is short. A wide range of12x x 32d 12y y 0.2 0.8 32d

of which adopt the perovskite structure, are candi- materials, prepared by this method, has been reporteddate materials for components of electrochemical [4]. The production of high T superconductingc

devices; e.g. as cathodes in solid oxide fuel cells oxides has been particularly successful.(SOFC) and the membrane material in pressure The physical and electrical properties of manydriven ceramic oxygen generators (COG) [1]. In the perovskites, with a general formula of ABO , are36d

case of pressure driven COGs, the quantity of dependent on the exact A/B and (A 1 B)/O ratios.oxygen produced can be maximised within certain The composition of the LSC and LSCF targetlimitations, by decreasing the thickness of the mem- materials, prepared by standard ceramic techniques,brane [2,3]. Therefore, there is a great interest in can be controlled with precision. However, theproduction of thin dense films of membrane material. repeated heating–cooling cycles experienced by the

Pulsed laser ablation has emerged as a promising target during ablation are reported to lead to incon-gruent melting, giving rise to compositional changesin the target, and subsequently, the deposition of*Corresponding author. Avdeling for Materialutvikling, Forskn-

ingssenteret, Norsk Hydro ASA, N-3907, Porsgrunn, Norway. sub-stoichiometric films [5].

0167-2738/00/$ – see front matter 2000 Published by Elsevier Science B.V.PI I : S0167-2738( 00 )00720-7

120 D. Waller et al. / Solid State Ionics 134 (2000) 119 –125

We have investigated how changes in two key 3. Resultsparameters in the laser ablation process; namely,pulse duration and the ablation chamber oxygen 3.1. LSC filmspartial pressure, effect the quality of LSC and LSCFfilms deposited onto silicon substrates. The XRD patterns of the LSC films and an LSC

standard are shown in Fig. 1. Ablation with theNd-YAG laser in a high oxygen pressure leads to theformation of a crystalline perovskite film. The crys-tallinity was not sufficient to determine unambigu-

2. Experimental methods ously whether the film is a rhombohedrally distortedperovskite, as in the standard. The film exhibits

The laser ablation systems consist of stainless steel limited preferred orientation as the (012) and (024)UHV chambers in which the oxygen partial pressures peaks are more intense than the (110/104) peaks,can be accurately controlled. Two laser systems were which is the most intense peak in the LSC standard.used. The first was an LPX-300I KrF excimer laser The film produced by Nd-YAG ablation, in an

21and the second was a Quantel frequency quadrupled oxygen pressure of 12 3 10 mbar (Fig. 1b), isNd-YAG laser. The laser fluences were measured barely crystalline. The presence of a weak peak atwith Gentec and Molectron energy meters. The 2u ¯ 478, suggests that the film is a perovskite, withparameters of these lasers are described in Table 1. (024) preferred orientation.

Dense LSC and LSCF targets, of 13 mm diameter, The XRD of the LSC films produced by excimerwere prepared using solid state methods from pure ablation, at both high and low oxygen partialoxide starting materials. The LSC and LSCF films pressures, show the presence of poorly crystallinewere deposited onto silicon wafer substrates, with a cubic phases. The phase deposited at an oxygen

24(100) orientation. The silicon substrates were heated partial pressure of 1 3 10 mbar, having a lattice˚to 7008C with either a quartz lamp or SiN resistive parameter of 6.31 A and the phase deposited at an

21heaters. The native oxide layer on the silicon sub- oxygen pressure of 1 3 10 mbar, having a lattice˚strates was not removed prior to deposition. The parameter of 6.19 A. Identification of these phases

optimum substrate temperature was determined in an was not possible.earlier study [6]. Oxygen partial pressures of 1 3

21 2410 and 1 3 10 mbar referred to as high and low, 3.2. LSCF filmsrespectively, were used during film deposition.

The quality of the films was determined from The XRD patterns of the LSCF films are shown inX-ray diffraction (XRD), using a Philips PW1710 Fig. 2. Ablation with the Nd-YAG laser, at both highdiffractometer in the Bragg–Brentano geometry. Cu and low oxygen partial pressures, leads to the growthK radiation was used and the diffractometer was of crystalline perovskite films, with a dominanta

equipped with a graphite secondary monochromator. (012) /(024) orientation. Excimer ablation of theThe surfaces of the targets were examined prior to LSCF target at an oxygen partial pressure of 1 3

21and after ablation, to investigate changes induced by 10 mbar leads to the deposition of a perovskiteablation. film with a (202) /(006) orientation. At an oxygen

Table 1Laser parameters

Laser l (nm) Power Pulse duration Pulse repetition Spot size22(J cm ) (FWHM) (ns) rate (Hz) (mm)

Excimer 248 2.0–5.0 25 10 331Nd-YAG 266 0.5–4.0 3.5 10 1 (circle)

D. Waller et al. / Solid State Ionics 134 (2000) 119 –125 121

Fig. 1. La Sr CoO films on silicon (100).0.7 0.3 3

Fig. 2. La Sr Co Fe O films on silicon (100).0.7 0.3 0.2 0.8 3

122 D. Waller et al. / Solid State Ionics 134 (2000) 119 –125

24pressure of 1 3 10 mbar, excimer ablation leads to changes. We observe that the perovskite peaks of thethe formation of a non-perovskite film, the structure targets have shifted to a slightly lower 2u value,of which is not determined. indicating an expansion of the lattice. The rhom-

bohedral distortion of the lattice, as evidenced by the3.3. LSC and LSCF targets presence of splitting of peaks at 2u ¯ 41, 58, and

528, is also reduced. A weak line at 2u ¯ 318, and inThe LSC and LSCF targets were examined using the case of the LSC target, at 44 and 528 are

XRD prior to and at the end of the ablation experi- observed. In general, the Nd-YAG-ablated targetsments. Two remarks should be made before the exhibit less modification than the excimer-ablatedresults of the XRD examination of the ablated targets targets.are presented. Firstly, the XRD geometry used in this The excimer and Nd-YAG lasers have similarstudy is not ideal for observing changes close to the wavelengths and power densities. Their main differ-surface as the bulk of the information contained in ence is the pulse duration. Calculations were made tothe diffraction pattern is obtained from depths greater determine the temperature of the perovskite targetsthan 10 mm from the surface. Secondly, the laser during ablation as a function of time and distancebeam only impinged on a proportion of the target from the surface [7]. Although all the thermalsurface (10% of the surface for the Nd-YAG laser parameters for the LSC and LSCF targets were notand 30% of the surface for the excimer laser). known, best estimate values were used. Tempera-

The XRD patterns of the LSC and LSCF targets, ture–time profiles for the Nd-YAG and excimerprior to and after ablation, are shown in Figs. 3 and ablation are shown in Figs. 5 and 6, respectively. For4, respectively, and the crystallographic data is the Nd-YAG ablation, a maximum temperature ofreported in Table 2. The fresh targets both possess 4800 K is reached within 1 ns. However, cooling isthe rhombohedrally distorted perovskite structure. very rapid so a temperature of 1000 K is neverAfter ablation, the targets exhibit some subtle exceeded at depths greater than 0.2 mm. For excimer

Fig. 3. La Sr CoO targets after ablation.0.7 0.3 3

D. Waller et al. / Solid State Ionics 134 (2000) 119 –125 123

Fig. 4. La Sr Co Fe O target after ablation.0.7 0.3 0.2 0.8 3

Table 2Crystallographic data for targets prior to and after ablation

3a a b˚ ˚ ˚Sample Crystal system a (A) c (A) Unit cell volume (A )

LSC bulk Rhombohedral 5.421 13.17 335.16 (55.86)LSC target Nd-YAG Rhombohedral 5.431 13.18 336.66 (56.11)LSC target excimer Cubic 3.845 56.89LSCF bulk Rhombohedral 5.443 13.26 392.82 (65.47)LSCF target Nd-YAG Rhombohedral 5.447 13.27 393.66 (65.61)LSCF target excimer Rhombohedral 5.451 13.28 394.56 (65.76)

a Hexagonal axes reported.b Brackets refer to the hexagonal cell volume/6 to compare with cubic cell volume data.

ablation, the maximum temperature achieved does ablation. Production of films with the perovskitenot exceed 2500 K, but the length of time and the structure using the excimer laser was only possibledepth of the high temperature zone exceeds that of with an LSCF target at high oxygen pressures. Nd-an Nd-YAG ablated target. YAG ablation allows the LSCF perovskite films to

be produced at both high and low oxygenpressures, and LSC films to be produced at high

4. Discussion oxygen pressures.Dam et al. have reported that structural modi-

It is apparent that the choice of laser effects the fication of a target during ablation can lead to poorstructure and crystallinity of the films produced by quality films [5]. In that example, incongruent melt-

124 D. Waller et al. / Solid State Ionics 134 (2000) 119 –125

Fig. 5. Calculated temperature-time profiles in target during excimer laser ablation.

Fig. 6. Calculated temperature-time profiles in target during Nd-YAG laser ablation.

ing of a YBa Cu O target led to phase separation At high temperatures, the LSC and LSCF targets2 3 7

and the formation of Y BaCuO and Y O . An become non-stoichiometric because of oxygen loss2 5 2 3

alternative target modification process is proposed. from the lattice. This results in an expansion of the

D. Waller et al. / Solid State Ionics 134 (2000) 119 –125 125

lattice, and in the case of rhombohedrally distorted iron-based perovskite oxides. It is possible toperovskites a relaxation of the structure allowing a produce films of the iron-based LSCF perovskite atcubic symmetry to be adopted [8]. Therefore, the lower oxygen partial pressures than the cobalt-basedreduction in the distortion of the perovskite structure LSC perovskites. Ablation of the LSCF targets withand the expansion of the lattice may be assigned to the excimer laser can lead to the formation ofoxygen loss from the lattice. perovskite films, albeit at high oxygen partial

It is well established that at high temperatures, in a pressures. Production of LSC perovskite films usinglow oxygen pressure environment, perovskites such the excimer laser was not possible, under the con-as LSC and LSCF are susceptible to reduction. The ditions employed. We believe that these observationsfollowing processes may occur at decreasing oxygen are a reflection of (1) reduction-induced phasepressures [9]: segregation of the target surface, particularly for

excimer ablation and (2) the greater stability of1]2 LnTmO → Ln TmO 1 TmO 1 O (1)3 2 4 22 LSCF in low oxygen partial pressure environments,

when compared with LSC.1]TmO → Tm 1 O (2)22 These results suggest that lasers, with long pulse

duration, are not suitable for the production of films1]Ln TmO → Ln O 1 O 1 Tm (3)2 4 2 3 22 consisting of materials that are susceptible to reduc-

tion. Although the Nd-YAG laser produces highwhere Ln is a lanthanide and Tm is a transition metaltarget temperatures, the period of time at high(e.g. Fe, Co, Ni, Mn). It is reported that cobalt-basedtemperatures, the short pulse duration results in lessperovskites are more susceptible to reduction thanstructural changes in the target surface than theiron-based perovskites [9]. The most intense diffrac-longer pulse duration a target is subjected to duringtion peak of La CoO coincides with the weak peak2 4excimer ablation.that is observed at 318 in the ablated targets [10].

The weak peaks at 2u ¯44 and 528, may be due tothe presence of a small quantity of the face centredcubic allotrope of metallic cobalt, which is the high

Referencestemperature form [11]. The XRD results suggest thatduring ablation, the surface regions of the perovskite

[1] B.C.H. Steele, in: U. Bossel (Ed.), Proceedings of the Firsttargets may be partially reduced. This is not surpris-European Solid Oxide Fuel Cell Forum, Lucerne, 1994, p.ing considering that ablation is occurring in a low375.oxygen partial pressure environment and that the

[2] S. Dou, C.R. Mason, P.D. Pacey, J. Electrochem. Soc. 132target surface exceeds 10008C. (1985) 1843.

Nd-YAG ablation leads to perovskite film forma- [3] H.J.M. Bouwmeester, H. Kruiydhof, A.J. Burgraaf, Solidtion for LSCFs and LSCs, even at low oxygen partial State Ionics 72 (1994) 185.

[4] F. Beech, I.W. Boyd, in: I.W. Boyd, R.J. Jackman (Eds.),pressures. The calculated temperature–time profile,Photochemical Processing of Electronic Materials, Academicfor excimer ablation, indicates that the surface regionPress, 1991.of the target reaches temperatures in excess of

[5] B. Dam, J.H. Rector, J. Johansson, S. Kars, R. Griessen,10008C; conditions that, if the reduction is kinet- Appl. Surf. Sci. 96–98 (1996) 679.ically controlled, may lead to a greater degree of [6] L.G. Coccia, G.C. Tyrrell, J.A. Kilner, D. Waller, R.J.reduction of the target, than during Nd-YAG abla- Chater, I.W. Boyd, Appl. Surf. Sci. 96–98 (1996) 795.

[7] R.K. Singh, O.W. Holland, J. Naroyan, J. Appl. Phys. 68tion.(1990) 233.

[8] T. Arakawa, N. Ohara, J. Shiokawa, J. Mater. Sci. 21 (1986)1824.

5. Conclusions[9] T. Nakamura, G. Petrow, L.J. Gauckler, Mater. Res. Bull. 14

(1979) 649.Oxygen pressure and laser pulse duration are key [10] A. Rabeau, P. Eckerlin, Acta Cryst. 11 (1958) 304.

factors in the successful production of cobalt- and [11] J.C.P.D.S. File 15-806.